fix(dialer): add switchNetns stubs for all non-Linux platforms

The switchNetns function was only defined for linux (dialer_netns.go,
build tag linux && !android) and android (dialer_android.go, stub).
This left darwin, freebsd, openbsd, windows, and other platforms with
an undefined symbol at dialer.go:55.

Add no-op stubs to all remaining platform files so the dialer package
compiles on every GOOS.

internal/net/dialer/dialer_darwin.go

@@ -35,3 +35,8 @@ func bindDevice(network, address string, fd uintptr, ifceName string) error {
 func setMark(fd uintptr, mark int) error {
 	return nil
 }
+
+// switchNetns is a no-op stub — network namespace switching is a Linux-only feature.
+func switchNetns(name string) (restore func(), err error) {
+	return nil, nil
+}